Re: [Haskell-cafe] Maybe, maybe not.

27 Jan 2010


      Ivan Miljenovic wrote:
...
2010/1/27 Tony Morris :
...
It might be more obvious by giving:
fromMaybe :: a -> (a -> x, x) -> x
I actually found this more confusing, and am not sure of its validity:
should that be "Maybe a" there at the beginning?
Sorry a mistake. Correction: fromMaybe :: a -> ((a -> x, x) -> x) -> x

{-# LANGUAGE RankNTypes #-}

data Maybe' a = M (forall x. (a -> x, x) -> x)

to :: Maybe' t -> Maybe t
to (M f) = f (Just, Nothing)

from :: Maybe a -> Maybe' a
from (Just a) = M (flip fst a)
from Nothing  = M snd


-- 
Tony Morris
http://tmorris.net/